When beech forests seed heavily, known as a mast, they can produce up to 15,000 seeds per square metre or 250 kg of seeds per hectare. This provides a bounty of food for native birds and insects. But it also feeds rodents, whose populations grow rapidly and in turn fuel an explosion of stoat numbers. This can be devastating for native species if predators are not controlled.
By forecasting a mast and monitoring tree flowering, we can be prepared to act. We use climate modelling to forecast when a beech mast will happen. We then use satellite imagery to see the extent of beech flowering across the country. This indicates where seed is likely to fall in the coming months.
We use this information to determine what forest sites and threatened species will likely need more protection from predators. We also monitor predator numbers to inform the timing of predator control.
Predicting the timing and intensity of mast events
Climate modelling
We use climate modelling to predict forest masts a year in advance.
Each year we use NIWA’s summer temperature data to predict the likelihood of beech forests seeding, in the following year.
When the summer temperatures during January, February and March are warmer than the previous summer, it is likely beech trees will flower the following spring. This means they will drop seed the following autumn. The difference in summer temperature is called the DeltaT. When the DeltaT is high, seeding is likely.
We map DeltaT over our beech forest areas across New Zealand each year. See the map of our mast prediction for 2026 and for previous years below.
Mast prediction for 2027
The Delta T indicates there will be no flowering and seeding of beech trees in the South Island in 2027. This forecast follows the wide-spread beech mast that occurred across South Island beech forests in 2026.
There are signs of possible light beech forest seeding in some North Island conservation areas such as Raukumara, Whirinaki, Rangataua (southern Ruapehu) and northern Ruahine.
The absence of a beech mast in the South Island in 2027 is a welcome relief for DOC's predator control team. We will still be undertaking the second phase of our 2026 mast response at key mast sites in 2027, as well as catching up on delayed work at non mast sites, particularly in the North Island.
The map below shows the beech mast prediction for 2027 using the difference in summer temperatures.
Map showing likely mast areas for 2027 | See larger (JPG, 404K)
Past predictions
The largest beech mast event in 7 years is occurring in 2026. Prior to this, DOC responded to the biggest beech mast in 40 years in 2019. Other significant mast events also occurred in 2016 and 2014.
Maps of mast forecasts for all years since 2018:
